Epithelia serve as a selective permeability barrier between tissue compartments. The uptake and transport of macromolecules by epithelial cells is a key aspect of their function, particularly in the gastrointestinal tract where antigens are sampled from the intestinal lumen and secretory antibodies transported from the lamina propria into mucosal secretions. The movement of macromolecules within and across cells is mediated by vesicular carriers, whose formation and targeting are specific, vectorial and highly regulated. Key components of vesicular transport pathways in all cells are small GTPases of the Ras superfamily. Among these, the Rab proteins appear to function in the targeting and-fusion of vesicles with their appropriate membrane destination. More than 30 Rab proteins have been identified to date. Two of those, Rabl7 and Rab25 are expressed exclusively in epithelia, and are presumably involved in specific vectorial transport processes that are unique to these cells. Rab25, the focus of this proposal, was originally isolated from gastric parietal cells and has been subsequently found to be expressed in virtually all epithelia. Immunohistochemistry has suggested that Rab25 is localized to a population of apically distributed intracellular vesicles similar in size and distribution to apically distributed intracellular vesicles similar in size and distribution to apical endosomes, however its function remains unknown. We propose to define the role of Rab25 in polarized epithelial cells, and the subcellular compartments to which it localizes. Additionally, we attempt to detect and characterize proteins that are associated with Rab25.